Natural gas generally refers to rarefied or gaseous hydrocarbons (comprised of methane and light hydrocarbons such as ethane, propane, butane, and the like) which are found in the earth. Non-combustible gases occurring in the earth, such as carbon dioxide, helium and nitrogen are generally referred to by their proper chemical names. Often, however, non-combustible gases are found in combination with combustible gases and the mixture is referred to generally as “natural gas” without any attempt to distinguish between combustible and non-combustible gases. See Pruitt, “Mineral Terms-Some Problems in Their Use and Definition,” Rocky Mt. Min. L. Rev. 1, 16 (1966).
For most markets, it is also desirable to minimize the presence of non-combustibles and contaminants in the product gas, such as carbon dioxide, helium, nitrogen and hydrogen sulfide. Depending on the quality of a given natural gas reservoir (which may contain as much as 50% to 70% carbon dioxide), the natural gas may be pre-processed at a natural gas plant for pre-removal of such of the above components.
Natural gas is widely recognized as the most important fuel gas in the United States and it provides more than one-fifth of all the primary energy used in the United States. Natural gas is also used extensively as a basic raw material in the petrochemical and other chemical process industries.
The composition of natural gas varies widely from field to field where it is produced from subterranean reservoirs. For example, a raw gas stream may contain as much as 95% methane, with only minor amounts of other hydrocarbons, nitrogen, carbon dioxide, hydrogen sulfide or water vapor. On the other hand, raw gas streams that contain relatively large proportions of heavier hydrocarbons and/or other contaminants are common. Before the raw gas can be sent to a supply and/or transportation pipeline, it must usually be treated to remove at least a portion of one or more of these contaminants.
For example, in some reservoirs, wellhead gas can contain high enough percentages of CO2 and H2S (acid gas) that the gas cannot be sold. Historically, amine plants have been utilized to sweeten the gas to meet pipeline specifications, using methods such as those disclosed for example in U.S. Reissue Pat. No. 29,428. Amine plants are large, heavy, and require significant supplies of outside electricity and de-ionized water. In addition, they generate an acid gas waste stream that typically must be supplemented with valuable fuel gas for flaring. This can also result in release of significant quantities of so-called “greenhouse gases” such as carbon oxides.
As an alternative to amine plants, membranes have been used for acid gas and other contaminant removal (such as nitrogen and other non-condensable gases), as well as higher hydrocarbons such as C3+, with limited success over the past twenty years or so. For example the problem of upgrading raw gas in the field, such as to sweeten sour gas, is described in U.S. Pat. No. 4,370,150, to Fenstermaker, the teachings of which are incorporated herein by reference. In this patent, the patentee describes a process that uses a membrane, selective for hydrogen sulfide and/or heavier hydrocarbons over methane, to treat a side stream of raw gas. The process is said to produce a membrane residue stream of fuel quality. The contaminants pass preferentially through the membrane to form a low-pressure permeate stream, which is returned to the main gas line upstream of the field compressor.
Membranes selective for removal of acid gas components (CO2) from gas streams, including methane, are disclosed in U.S. Pat. No. 4,963,165. Use of membrane technology for removal of acid gases is also generally disclosed in U.S. Pat. Nos. 4,466,946; 4,529,411; 4,561,864; 4,639,257; 5,256,296; 5,401,300; 5,407,466; 5,407,467; 6,053,965; and 6,161,386. The removal of nitrogen contaminant from natural gas by use of membranes is disclosed in U.S. Pat. Nos. 5,669,958 and 6,035,641. Use of membranes for removal of C3+ hydrocarbons from natural gas streams is also disclosed in U.S. Pat. No. 6,161,386. A two-stage membrane separation process is disclosed in U.S. Pat. No. 5,256,295. The teachings of each of the foregoing patents are incorporated herein by reference in their entirety.
As the raw gas stream travels from the wellhead to a gas processing plant and ultimately to a pipeline, gas generally passes through compressors or other field equipment. These units require power to operate, and it would be desirable to run them using gas engines fired by natural gas derived from the field. A related operation is to use such natural gas as combustion fuel for turbines, which are then used to drive other equipment, for example, electric power generators and compressors. However, since the field gas has not yet been brought to specification, however, this practice may expose the engine to fuel that is of overly high Btu value, low octane number, or corrosive. Also it would be desirable to not use natural gas from which contaminants have been removed, i.e., pipeline gas, as it has more value in the marketplace. In these cases, the water and hydrocarbon dewpoints of the gas should be below the lowest temperature likely to be encountered en route to the turbine. If this is not done, the feed stream may contain entrained liquid water and hydrocarbons. These do not burn completely when introduced into the turbine firing chamber, and can lead to nozzle flow distribution problems, collection of liquid pools and other reliability issues. Additionally high concentrations of heavy hydrocarbons tend to make the fuel burn poorly, resulting in coke formation and deposition of carbon in the fuel pathways and on the turbine blades. These deposits reduce turbine performance and affect reliability.
A number of patents mention treatment of raw natural gas for purposes of obtaining fuel gas that may be used as fuel in gas turbines for electric power generation. For example, U.S. Pat. Nos. 6,053,965 and 6,161,386 disclose use of a separation membrane to upgrade raw natural gas to run field engines and generate electrical power therefrom. Similar processes are disclosed in U.S. Pat. Nos. 6,298,652 and 6,684,644. U.S. Pat. No. 6,035,641 discloses use of a membrane to upgrade gas containing large amounts of nitrogen, followed by use of that gas as combustion fuel for a turbine that generates electric power. The patents referenced hereinabove are also incorporated herein by reference in their entirety.
While process equipment employing membranes can be more compact than amine plants and has been disclosed as a means to upgrade raw natural gas, as previously mentioned herein, such separation membranes are frequently subject to a number of undesirable characteristics, the most significant of which relate to relatively large methane losses and greenhouse emissions from disposal of the permeate gas. These characteristics result in a process that in many cases is inefficient and uneconomical.
As can be seen, it would be desirable to develop a gas sweetening method and system which can more effectively remove undesirable contaminants from natural gas streams without the large capital, electrical power, and water requirements of an amine plant, or the undesirable methane losses and high emissions of greenhouse gases into the environment that are associated with conventional membrane separation processes.